The invention relates generally to reduction of water consumption and improved energy efficiency in chemical processes used in the distillation of industrial chemicals, and in particular is suitable-for reduction of water consumption in the production of purified terephthalic acid.
Terephthalic acid is useful in a diverse variety of industrial applications and chemical processes. For example, terephthalic acid is a starting material for producing polyesters including plastic and Dacron™ polyester used in textile and container production. Polyethylene terephthalate (PET) is a form of polyester or Mylar™ that is an extremely tough resin and useful in many industrial and consumer applications. Soft drink and water bottles are made from this resin in addition to plastic jars and clamshell packages used in consumer good transport and food distribution. Purified terephthalic acid is a higher grade of terephthalic acid which is used for finer industrial applications.
Terephthalic acid typically is produced by reaction of paraxylene with molecular oxygen in the presence of a catalyst. During the production process, acetic acid is used as a solvent of terephthalic acid. The acetic acid becomes diluted in water during the oxidation in a reactor section of a terephthalic acid plant in the production cycle. A portion of the acetic acid and water containing stream is then sent to a dehydration unit to recover acetic acid for recycle back to the reactor and to remove the water generated in the reactor for use in the pressure filter or waste treatment.
Various different approaches have been employed in the terephthalic acid plants to separate the acetic acid and water. Typically, in prior art systems both make up water and water generated by the reaction ultimately are sent to a wastewater treatment facility for safe disposal.
One approach for separation of water and acetic acid is by conventional distillation based on the differences in boiling point of the components. Distillation has been widely used as a primary unit operation for acetic acid recovery from water. In such processes, one or more towers are utilized to process a number of streams of varying concentrations of acetic acid. However, the distillation of acetic acid and water is very inefficient (i.e. energy intensive) due to the close-boiling characteristics of the acetic acid/water system.
There has been an effort to look for alternative processes to minimize the high operating costs associated with the conventional distillation for the separation of acetic acid and water. Chemical processors and companies have resorted to azeotropic distillation involving the addition of selective alkyl acetate, such as isobutyl acetate, normal butyl acetate, normal propyl acetate, etc., as a solvent to the azeotropic dehydration column.
Another approach is azeotropic distillation where extraction solvents (also termed entrainers) are used to form azeotropes with the acetic acid and water providing a change in energy requirements for processing.
More specifically, the solvent forms a low boiling azeotrope with water and therefore improves the relative volatility for the separation between the acetic acid containing stream and the alkyl acetate/water azeotrope. This reduces the energy and theoretical stage requirements for the same separation. Compared to the conventional distillation, an azeotropic distillation approach typically reduces the energy (i.e. steam) consumption by 20-40% at the acetic acid/water dehydration column while giving relatively low acetic acid concentration, 300-800 ppm, in the distilled water. The azeotropic distillation column is generally employed in the terephthalic acid manufacturing plants in all prior art systems. However, the effluent water stream is not sufficiently clean for use in the terephthalic acid purification unit, and so the water is sent for disposal with the organics contained therein.
A combination of liquid-liquid extraction with azeotropic distillation is yet another approach for acetic acid and water separation. In particular, a method used in terephthalic acid production includes the use of liquid-liquid extraction with an extraction solvent into which acetic acid is extracted thus substantially reducing the concentration of acetic acid in the aqueous stream. Typically, the extraction solvent is selected from a group of isobutyl acetate, normal butyl acetate, isopropyl acetate and normal butyl acetate. The resulting extract comprises mainly the solvent and acetic acid, and also contains a lesser amount of water, and so this stream is easier to separate in an azeotropic distillation column. An azeotropic mixture comprising water and an ester that is also the extraction solvent is distilled from the top of the azeotropic distillation column, and an acetic acid rich liquid stream is recovered from the bottom of said azeotropic distillation column.
Such extraction and azeotropic distillation processes for recovery of acetic acid from aqueous streams are described by, for example, Othmer in U.S. Pat. No. 2,395,010 (1946) and Sasaki et al. in U.S. Pat. No. 5,662,780 (1997), and have been applied to recovery of acetic acid from manufacture of terephthalic acid as described, for example, by Ohkoshi et al. in Japanese Patent Application JP 244196/95 (1995) and European Patent Application EP 0 764 627 (1995).
More specifically, Ohkoshi et al. in European Patent Application EP 0 764 627 A1 (also Japanese Patent Application JP 244196/95) describe yet another system for recovery of acetic acid from aqueous streams in a terephthalic acid manufacturing process. In this process there is an additional liquid-liquid extraction column to process terephthalic acid mother liquor using an extraction solvent recovered from the azeotropic distillation column. The organic extract is then sent directly to the azeotropic distillation column and the aqueous raffinate to a wastewater treatment plant.
However, the process with each of these [three] approaches still requires considerable amounts of energy.
To further reduce energy consumption in recovery of acetic acid from such streams, Jang et al. in our co-pending U.S. patent application Ser. No. 12/382,801 (2009), describe a system and method for acetic acid dehydration in which there is a liquid-liquid extraction column to which water-rich feed streams are fed, having a guard bed situated near the top within the extraction column for conversion by reaction with acetic acid of alcohol within the mixture to the corresponding ester; and an azeotropic distillation column to remove residual water from acetic acid, to which water-poor feed streams are fed directly at a height of the azeotropic distillation column at which the mixture therein has a similar water concentration. The liquid-liquid extraction column produces an extract comprising an extraction solvent and acetic acid which is sent to the azeotropic distillation column to remove residual water. The acetic acid from the bottom of the azeotropic distillation column is sufficiently pure for reuse in the oxidation reactor and the water after stripping of the solvent is sent to wastewater treatment.
Nevertheless, even with these recent improvements, there is still a requirement for large amount of fresh process water, about 2.1 tonne make up water per 1 tonne pure terephthalic acid production. As a result, a considerable quantity of wastewater is discharged.
Accordingly, it is desirable to further improve the above processes, and thereby effect a reduction in consumption of energy and water, reduction in loss of organic materials, and reduction in aqueous effluent, which the present invention addresses.